This invention relates to zinc titanate pigments and to spacecraft thermal control coatings utilizing such pigments.
Long term operation of vehicles and equipment in the space environment requires the availability of effective thermal control coatings. In the absence of a stable white coating or other protective measures, surfaces which are exposed to solar radiation would rapidly become overheated, largely because of absorption of solar energy. Spacecraft equipment would eventually manfunction when its temperature limitations were exceeded. Coatings for such applications should exhibit a very low and stable ratio of solar absorptance (.alpha.s) to infrared emittance (.epsilon.) along with good mechanical integrity and ease of application.
Various problems have been presented in the development of pigments and coatings for space applications. Many pigments which have favorable reflectance properties for terrestrial uses undergo a severe degradation of reflectance upon prolonged exposure to the combined conditions of vacuum and ultraviolet radiation encountered in outer space. Most of the binders and vehicles used for conventional coatings cannot be used in space because they are rapidly damaged by solar ultraviolet radiation. In some cases pigments and binders which are stable to the space environment may be incompatible with one another.
Progress has been made in recent years in determining degradation mechanisms and providing stable binders and pigments for space applications. For example, U.S. Pat. Nos. 3,576,656 and 3,607,338 disclose methods of treating zinc oxide and zinc orthotitanate pigments, respectively, to improve their initial reflectance and resistance to loss in reflectance upon exposure to ultraviolet radiation in vacuum. A need exists, however, for further improvement in pigment properties, particularly for zinc orthotitanate, which exhibits higher intrinsic reflectance than the zinc oxide and titanium oxide from which it has been conventially prepared by solid-state reactions. One source of pigment damage and loss of stability has been the grinding or milling steps required to obtain the very small particle size (submicron to 5 microns, and primarily about 0.6 micron) needed for optimum reflectance of solar radiation. Zinc orthotitanate prepared by previous zinc oxide-titanium oxide reactions has a much larger particle size, and the prolonged grinding or milling required to obtain finer particles results in decreased reflectance owing to particle damage (commonly known as "mill yellowing"). In addition, contaminating impurities are inherently introduced, and degradation resistance is decreased owing to the creation of active sites for degradation mechanisms. Initial formation of pigment particles in the desired size range would alleviate these problems. Another problem encountered with zinc orthotitanate pigments is that they frequently show decreased reflectance at wavelengths below 380 nm, indicating absorption by small amounts of uncombined zinc oxide.